a) Field of the invention
This invention relates to a combined eye- and waist-level real image mode finder suitable for zooming compact cameras.
b) Description of the prior art
In general, a real image mode finder for compact cameras, as represented by an optical system set forth in Japanese Patent Preliminary Publication No. Sho 61-156018 and as shown in FIG. 1, comprises an objective lens system 1 adapted to form an image for once, an eyepiece system 2 adapted to magnify the image, and an image erecting prism 3 arranged between the systems 1 and 2 so that an eye is placed at an eye point set in a position separated by about 10-20 mm from the exit portion of the eyepiece 2 to view a virtual image, thereby confirming a visual field for photographing.
It is necessary for this finder, however, to place the pupil of the eye within a range of about 2-5 mm in diameter at the eye point set in the position located rearward by about 10-20 mm on an optical axis from the eyepiece in order to view the field of a picture to be photographed with a favorable visual field factor, imaging performance, and diopter. Consequently, under a status devoid of space to place the eye at the eye point and in a case where sudden shutter chance pressed for time has been given, it is impossible to view the field of the picture to be photographed with a favorable visual field factor, imaging performance, and diopter.
Therefore, in order to eliminate such a defect, a combined eye- and waist-level finder has been proposed and, for example, the finder described in Japanese Patent Publication No. Sho 59-11089, as depicted in FIG. 2, is composed of an objective lens 4, a half mirror 5, a reflecting mirror 6, an eye-level eyepiece 7, and a waist-level eyepiece 8. Further, the finder set forth in Japanese Patent Preliminary Publication No. Sho 63-262635, as shown in FIG. 3, is constructed of an eye-level objective lens 9, a waist-level objective lens 10, a Dach prism 11, an eye-level eyepiece 12, and a waist-level field lens 13.
In the finder of Sho 59-11089 described above, however, since light of a waist-level image is such as to be reflected once downward from the half mirror 5 and further reflected from the reflecting mirror 6 toward the waist-level eyepiece 8 with the result that it is involved twice in the half mirror 5 and also reflected by the reflecting mirror 6, the waist-level image will become very dark in practical use. Further, this finder has a disadvantage that the half mirror 5 is configured as a plane-parallel plate and as such a twin image is produced by reflected light of both sides of the plate. In addition, the finder has another disadvantage that since it is an albada type finder in which a real image is not formed, the diopter of a field image is difficult to coincide with that of a field mask, namely, the field mask is difficult to be set.
Also, the finder of Sho 63-262635 comprises an optical system in which a ray of light entering from the waist-level objective lens 10 is reflected from the Dach prism 11 and then formed as an image in the vicinity of the waist-level field lens 13 and an optical system in which a ray of light entering from the eye-level objective lens 9 traverses the Dach prism 11 to magnify the image by the eye-level eyepiece 12, and has a defect that since its entrance window is provided with separate objective lenses for the waist-level and the eye-level, space for the entrance window will increase. Further, the finder has another defect that, from the same reason, when it is used as a zoom finder, individual objective lenses need to be constructed as zoom lenses and the number of parts will increase. Also, it has a further defect that, from the same reason, parallax occurs between the eye-and waist-level systems.